This is a repository copy of Ultralow Phase Noise 10-MHz Crystal Oscillators. White Rose Research Online URL for this paper: https://eprints.whiterose.ac.uk/141563/ Version: Published Version Article: Everard, Jeremy Kenneth Arthur orcid.org/0000-0003-1887-3291, Burtichelov, Tsvetan Krasimirov and Ng, Keng (2019) Ultralow Phase Noise 10-MHz Crystal Oscillators. IEEE Transaction of Ultrasonics Ferroelectrics and Frequency Control. pp. 181-191. ISSN 0885- 3010 https://doi.org/10.1109/TUFFC.2018.2881456 Reuse This article is distributed under the terms of the Creative Commons Attribution (CC BY) licence. This licence allows you to distribute, remix, tweak, and build upon the work, even commercially, as long as you credit the authors for the original work. More information and the full terms of the licence here: https://creativecommons.org/licenses/ Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing [email protected] including the URL of the record and the reason for the withdrawal request. [email protected] https://eprints.whiterose.ac.uk/ IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 66, NO. 1, JANUARY 2019 181 Ultralow Phase Noise 10-MHz Crystal Oscillators Jeremy Everard , Tsvetan Burtichelov, and Keng Ng Abstract— This paper describes the design and implementation noise onto the carrier which typically produces a 1/ f 3 phase of low phase noise 10-MHz crystal oscillators [using stress noise contribution in the oscillator. ∼ compensated (SC) cut crystal resonators] which are being used as Methods to reduce the drive-level dependence include, for a part of the chain of a local oscillator for use in compact atomic clocks. The design considerations and phase noise measurements example, cancelation of two opposing effects by operating a are presented. The design includes a low-noise transformer quartz crystal oscillator at a point slightly above the crystal coupled differential amplifier, spurious resonance rejection filter, series resonance where a change in oscillator phase would and electronically tuned phase shifter. Phase noise measurements result in a change in crystal drive level. This produces a − − demonstrate a performance of 122 dBc to 123 dBc/Hz at shift in crystal frequency exactly equal to but opposite to 1-Hz offsets and −148 dBc/Hz at 10-Hz offsets. The phase noise at 1-Hz offset is very similar to the phase noise produced by the frequency shift resulting from the resonator phase versus the low-noise version of a doubled 5-MHz BVA resonator-based frequency characteristic [2]. Another method to reduce drive oscillators (model number 8607) previously produced by Oscil- dependence uses multiple resonators to share the power [4]. loquartz. The noise floor of the oscillators presented in this The far from carrier noise floor is reduced by increasing the − paper is around 161 dBc/Hz. These designs can be used as crystal power so this should be considered at the same time the reference oscillator to control the timing of many modern electronics systems. as the drive-level dependence of the crystal [3]. The effect of resonator out-of-band impedance on the sus- Index Terms — Frequency control, low noise crystal oscillators, taining stage white noise should be considered [6]. Multiple low noise oscillators, noise, oscillators, phase noise. amplifiers with inter-amplifier attenuation can also be used to improve performance [7]. I. INTRODUCTION A variety of self-limiting amplifier/oscillator types are dis- HE phase noise and jitter in oscillators set the ultimate cussed in detail in [3], which highlight the requirement for Tperformance limits in communications, navigation, radar, high Q and adequate suppression of l/ f flicker-of-phase-type and precision measurement and control systems. It is, there- noise, and improvement in oscillator noise floor signal to fore, important to develop simple, accurate linear theories noise. A number of oscillator topologies are also discussed which highlight the underlying operating principles and to including the Pierce, Miller, Butler, and bridged-T configu- present circuit implementations based on these theories. rations. Measurements of the AM-to-PM conversion are also Crystal oscillators offer a solution for precision oscillators important [9]. due to the precise resonant frequency, very high Q,and However, there are very few papers (if any) showing com- controllable temperature coefficients. plete designs with phase noise near or below 120 dBc/Hz Many papers have been written on high frequency, very high at 1 Hz offset in 10-MHz crystal oscillators. Ultralow-phase− frequency, and ultra-high frequency bulk crystal and surface noise oscillators using Boîtier à Vieillissement Amélioré acoustic wave oscillators [1]–[10] including a significant tuto- (BVA) [12] stress compensated (SC) cut resonators have rial review of crystal oscillators [11]. been described [13], [14] but the detailed oscillator circuit Key aspects to be considered to achieve low phase noise in descriptions were not included. crystal oscillators are the 1/ f flicker noise of the amplifier, the The quoted phase noise for the low-noise version of the flicker-of-frequency noise in the resonator [1], and the ampli- BVA oven-controlled crystal oscillator (OCXO) 8607 oscilla- tude modulation to phase modulation (AM-to-PM) conversion tors in previous data sheets at 1-Hz offset is 130 dBc at at higher crystal drive power levels due to nonlinear effects in 5MHzand 122 dBc/Hz at 10 MHz. The phase− noises of the crystal [2], [3], [5]. There is transposition of this flicker the oscillators− described in this paper, which use standard SC cut resonators, are very similar to the doubled 5-MHz output Manuscript received August 6, 2018; accepted November 8, 2018. Date of ( 6 dB) and directly to the 10-MHz output. publication November 19, 2018; date of current version January 14, 2019. + This work was supported by EPSRC under Project EP/J500598/1 and Project A number of low phase noise commercial designs are EP/L505122/1, in part by Leonardo MW, Ltd. (formerly Selex ES Ltd.), in available, along with their phase noise specifications; however, part by BAE Systems, and in part by HCD Research, Ltd. (Corresponding author: Jeremy Everard.) circuit diagrams are not provided. For example, the prelimi- J. Everard is with the Department of Electronic Engineering, University of nary data sheet for the Morion MV3336M specifies 119 to York, York YO10 5DD, U.K. (e-mail: [email protected]). 120 dBc at 1-Hz offset so the oscillator presented− in T. Burtichelov was with the Department of Electronic Engineering, Univer- − sity of York, York YO10 5DD, U.K. He is now with CGC Technology Ltd., this paper is 2.5–3.5 dB better than the specification. The Basingstoke RG24 8WA, U.K. “extraordinary range” of low phase noise 10-MHz OCXOs K. Ng was with the Department of Electronic Engineering, University of manufactured by NEL states 120 dBc at 1-Hz offset. York, York YO10 5DD, U.K. He is now with Mars Wrigley Confectionery, − Slough SL1 4LG, U.K. The short and medium-term phase noise and Allan deviation Digital Object Identifier 10.1109/TUFFC.2018.2881456 of the local oscillator are limiting factors of the performance of This work is licensed under a Creative Commons Attribution 3.0 License. For more information, see http://creativecommons.org/licenses/by/3.0/ 182 IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 66, NO. 1, JANUARY 2019 A suitable model is shown in Fig. 1 [22]–[25]. This consists of an amplifier with two inputs which are added together. These represent the same input but are separated to enable one to be used to model the noise input and the other for feedback. The resonator is represented as an LCR circuit where any impedance transformation is achieved by varying the component values. This circuit, through positive feedback, operates as a Q multiplication filter. It also contains the additional constraint that the AM noise is suppressed in the limiting process. This means that the phase noise component of the input noise drops to kT/2 which has been confirmed by NIST [26] and this research group. This limiting also causes the upper and lower sidebands to become coherent and has Fig. 1. Oscillator model. been defined as conformability by Robins [27]. The model is put in this form to highlight all the effects, which are often not clear in a block diagram model. most systems including, for example, vapor cell atomic clocks. A general equation for the phase noise can be derived as Extremely low phase noise can be achieved by combining the shown in [24] and [25] which incorporate a number of operat- close to the carrier performance of crystal oscillators with the ing conditions including multiple definitions of output power medium offset and the low noise floor of a dielectric resonator from the amplifier, the input and output impedances, the ratio oscillator (DRO) [15] and also including narrowband digitally of loaded to unloaded Q factor (QL/Q0), and operating noise controlled direct digital synthesizers [16]–[18]. figure F. It is interesting to note that the DRO described in [15] The specific phase noise equation, where ROUT RIN and and [16] had similar or better phase noise performance than power is defined as the power available at the output= of the multiplied 100-MHz crystal oscillators, but the 10-MHz oscil- amplifier (PAVO ), simplifies to the following equation: lator was able to improve the performance and stability below 10-Hz offsets. The resulting system [16] is highly versatile 2 in terms of tuning and locking the flywheel frequency to the FkT f0 L( f ) 2 2 . (1) atomic resonance and is capable of providing multiple highly = 2 Q L QL f 8Q 1 PAVO stable output signals at both RF and microwave frequencies.